It has been known for a long time that various doped rare earth metal vanadates have luminescent properties. YVO4:Eu3+ is e.g. industrially used as a red luminescent material in cathode ray tubes or in colour television sets (U.S. Pat. No. 3,360,674) and fluorescence lamps. In its mono-crystalline form, it has also been used as a polarisator and as laser material (R. A. Fields et al., Applied Physical Letters 51, 1885, 1987).
The luminescent properties of yttrium vanadate doped with Tm, Tb, Ho, Er, Dy, Sm or In, of GdVO4:Eu and LuVO4:Eu are also known in literature (see Ullmanns Encyclopedia of Industrial Chemistry, Wiley-VCH, 6th ed., 1999, volume A15, Luminescent Materials and the publications cited therein). Further luminescent vanadates are e.g. known from U.S. Pat. No. 6,203,726.
In the development of lasers, GdVO4:Tm, Ho and GdVO4:Nd crystals have been used with a diode laser as an excitation source (see P. J. Morris et al., Opt. Commun., (Netherlands) 111, 439 (1994) and P. K. Mukhopadhyay et al., National Laser Symposium, CAT, Indore (India) 49 (Feb. 6-8, 1997)). GdVO4:Bi powder was proposed as a scintillation material in computer tomography (G. Leppert et al., Applied Physics A59, 69 (1994)). That the co-doping of europium-doped yttrium and gadolinium vanadates promotes the Eu3+ emission intensity and shifts the excitation wavelengths to longer wavelengths at the same time, whereby an excitation with UV-A becomes possible, is also known from literature (S. Z. Toma et al., J. Electrochem. Soc. 114,9 (1967), pages 953-955; R. K. Datta et al., J. Electrochem. Soc. 114, 10 (1967), pages 1058-1063 and B. N. Mahalley et al., Applied Physics A 70, 39-45 (2000)).
These vanadates are conventionally produced by mixing oxidic starting materials and their calcination at high temperatures, whereby a macro-crystalline material is obtained.
Many industrial applications, however, require the homogenous dispersion of the vanadates in liquid media (e.g. aqueous or organic solvents) or solid media (e.g. polymer materials). If the macro-crystalline material is to be transferred into a fine-crystalline material, additional process steps are required, such as milling and size selection. Not only is the yield of useful particles reduced thereby but this also leads to contaminations, e.g. by mechanical abrasion during the milling steps. These contaminations can also have negative effects on the quantum yield (ratio of emitted to absorbed photones).
For this reason, recently efforts have been undertaken to obtain nanoparticulate vanadates as a product of a direct synthesis. “Nanoparticulate” means that the diameter (measured at the longest axis for non-spherical particles) is less than 1 μm. In connection therewith, it is of particular interest to obtain nanoparticles having a diameter of less than 30 nm since they no longer interact with the light incident on a medium and the dispersion thus becomes transparent.
K. Riwotzky and M. Haase (J. Phys. Chem. B 1998, 102, 10129-10135) described for the first time the wet-chemical synthesis of doped colloidal nanoparticles of the formula YVO4:Ln (Ln=Eu, Sm, Dy). The synthesis starts with the corresponding metal nitrates and Na3VO4, which were dissolved in water and reacted for one hour at 200° C. in an autoclave. The process, however, leads to a broad size distribution of the vanadate nanoparticles and requires complex purification and size selection steps in order to isolate particles in the range of 10 to 30 nm. The yield of the nanocrystalline YVO4:Eu after dialysis thus only amounted to 3%. The authors indicate a quantum yield (ratio of the emitted photones to the absorbed photones) of 15% at room temperature in water for an yttrium vanadate doped with 5% Eu3+. This quantum yield is significantly below that of the macrocrystralline material which was ascribed to the essentially higher surface-volume ratio of nanoparticles in connection with luminescence quenching processes occuring at the surface.
A. Huignard et al. in Chem. Mater. 2002, 14, pages 2264-2269 describe the synthesis and characterization of YVO4:Eu colloides having a particle diameter of approximately 10 nm. The synthesis was performed by reaction of yttrium and europium nitrate in the corresponding molar ratio as well as sodium citrate and Na3VO4 in water. Various dialysis steps follow after 30 minutes of ageing of the solution at 60° C. The size distribution obtained was more narrow-than with. Riwotzky and Haase but nevertheless relatively broad with a standard deviation of 37.5% (average size 8 nm, standard deviation 3 nm). No exact indications were made on the particle yield. According to the authors, a quantum yield of 16% is obtained for europium-doped yttrium vanadates only with a content of europium of x=0.20 (20%). The authors assume that lattice defects which prevent the energy transfer are responsible for this.
The synthesis of nanoparticulate transition-metal oxide pigments, such as CoAl2O4, Cr2O3, ZnCo2O4, (Ti0.85, Ni0.05, Nb0.10)O2, α-Fe2O3 und Cu(Cr,Fe)O4, in diethylene glycol at 140° C. is described in C. Feldmann, Advanced Materials, 2001, 13, no. 17, pages 1301-1303. The average particle diameter of the pigments ranges between 50 and 100 nm.
Also non-doped vanadates are of interest for industrial applications. WO 02/072154 e.g. discloses the use of nanoparticulate GdVO4 as contrast medium in medical diagnosis methods based on NMR, such as the computer tomography. The GdVO4 synthesis described therein uses the methods of Riwotzky and Haase and therefore has the same disadvantages.
The object of the present invention was therefore to provide a new synthesis for nanoparticles comprising metal(III) vanadates which leads to a narrow particle size distribution without further size selection steps at a high yield.
According to a further aspect of the invention, a synthesis is intended to be provided which leads to nanoparticles comprising metal(III) vanadate which can be easily dissolved in water and alcohols, but also in aprotic organic media in accordance with a preferred embodiment.
It is a further object of the present invention to provide nanoparticles comprising metal(III) vanadates in a narrow particle size distribution which can easily be dissolved in water and alcohol, but also in aprotic organic media in accordance with a preferred embodiment.
According to a preferred embodiment and a further aspect of the technical object, luminescent nanoparticles comprising metal(III) vanadate with comparatively high quantum yields should further be provided.
According to one further preferred embodiment and aspect of the technical object, a synthesis method for nanoparticles is to be provided which can be easily conducted in larger scale.
Other technical objects can be derived from the following description of the invention.